COP 3275 – Character - PowerPoint Presentation

Slide1

COP 3275 – Character Strings and Introduction to Pointers

Instructor: Diego Rivera-GutierrezSlide2

Administrative stuff

Did you guys have a good break?

Quiz

#5

was due 2 minutes ago! (give or take)

Friday (7/3) is the observed day for Independence Day –

No class

No quiz this weekSlide3

Administrative stuff

Homework #4 is due Thursday.

How is that going?

Who has finished the interactive portion? (Receiving plays)

Who has finished coding the losing condition?

Who has finished the open procedure?

Anyone has worked on the extra credit?Slide4

Quick overview of last class

Character strings store

text

!

char *<identifier>;

//pointer

char <identifier>[<max-size>];

//

array

char *

str

= "this works fine";

printf

("%s",

str

);

Console arguments

int

main (

int

argc

, char **

argv

);

//more standard

int

main (

int

argc

, char *

argv

[]);

//easier to understandSlide5

Length of a string

int

length

(

char

*s

)

{

int

len

= 0;

while

(

s[

len

]

!= '\0'

) {

len

++;

}

return

len

;

}Slide6

Length of a string

int

length

(

char

*s

)

{

int len = 0; while(s[len]) { len++; } return len;}

Slightly more efficient!Slide7

User string input

Let’s say we have a piece of code like this one:

char *

str

;

scanf

("%s",

str

);

//notice that this one doesn’t need &

printf

("%s", str);What do you think will happen?Let’s test itSlide8

User string input

Ok, so we need to allocate the string.

char

str

[100];

//using array notation makes sense here.

scanf

("%s",

str

);

//notice that this one doesn’t need &

printf("%s", str);What happens when we try to read more than one string?Slide9

User string input

char

s1[100

];

//using array notation makes sense here

.

char

s2[100

];

//using array notation makes sense here.

scanf

("%s%s", s1, s2); //still we don’t use & for stringsprintf("%s\n", s1);printf("%s\n", s2);Slide10

Comparing strings

In the previous one. How do I know if the two strings are the same?

For example, the user inputs:

Independence

Independence

Could I compare

if(s1 == s2)

Why or why not?Slide11

Comparing pointer types

In general, == compares

values

.

“Values” when it comes to pointers refer to positions in memory

So when I do s1 == s2, the code I’m generating is:

Are s1 and s2 pointing to the same location in memory?

NOT

:

Are s1 and s2 storing the same value.

This is the same for arrays and pointers (wait for it…)Slide12

Comparing pointer types

So if I do:

int

arr1[2] = {0,1};

int

arr2[2] = {0,1};

The same problem arises. Slide13

Comparing strings

int

equal

(

char

*

s1,

c

h

ar *s2) { int i = 0; while(s1[i] != '\0' && s2[i

]

!= '\

0' &&

s1[

i

] == s2[

i

]

)

{

i

++;

}

return

s1[

i

] == '\0' && s2[

i

]

== '\0'

;

}Slide14

Comparing strings

int

equal

(

char

*

s1,

c

h

ar *s2) { int i = 0; while(s1[i] && s2[i] && s1[i

] == s2[

i

]

)

{

i

++;

}

return

s1[

i

] == '\0' && s2[

i

]

== '\0'

;

}Slide15

Comparing strings

int

equal

(

char

*

s1,

c

h

ar *s2) { int i = 0; while(s1[i] && s2[i] && s1[i

] == s2[

i

]

)

{

i

++;

}

return

!(s1[

i

] || s2[

i

]);}

Isn’t this one beautiful????

OK, OK, maybe it’s just meSlide16

Comparing strings

While understanding those functions is important to our understanding of strings.

Truth is we rarely have to write that particular function (or the length one for that matter).

<

string.h

> defines a lot of useful functions.

strcmp

– compares two strings (and does more that our equal function)

strlen

– finds the length of a string.Slide17

Comparing structs?

This produces a similar problem.

However, this is not allowed in compilation.

== as an operator, just doesn’t know how to compare non-standard types.

To compare

structs

, you need to define functions.Slide18

PointersSlide19

Ok… so what’s a pointer?

Let’s first talk about

“

indirection”